The numbers in brackets below refer to references listed in the Appendix, the teachings of which are hereby incorporated by reference.
Several large-scale demonstration projects supported by the U.S. Department of Energy (DOE) will generate coal gasifier solid waste containing calcium sulfide [1]. These projects involve integrated coal gasification, combined-cycle (IGCC) systems for generating electric power more efficiently than can be accomplished with present systems. Cooperative agreements have been reached with industry for undertaking these projects and preliminary design of the systems is underway. The systems are expected to be in operation soon. Successful demonstration of the IGCC concept could lead to the wide scale adoption of this technology for new power plants.
An important aspect of the IGCC concept is the use of lime to capture 90% of the sulfur produced during gasification of the coal. However, the waste calcium sulfide which is generated cannot be placed directly in a landfill where it would react slowly with moisture to release toxic hydrogen sulfide gas. Therefore, it has been proposed to oxidize the calcium sulfide to calcium sulfate for disposal, but the oxidation of calcium sulfide to calcium sulfate is not a straightforward operation. An earlier attempt to oxidize solid particles of calcium sulfide at temperatures ranging from 650.degree. to 980.degree. C. by using an oxygen partial pressure of 0.06 atm resulted in only partial conversion of the calcium sulfide [2]. It appeared that under these conditions an impenetrable shell of calcium sulfate formed around each particle stopping the reaction and leaving an unreacted core of calcium sulfide. To circumvent this problem it has been proposed to use dolomitic lime and only partial sulfidation of the material, but of course, much more lime will be required and much more waste will be generated than would be the case were the lime completely sulfided.
An alternative approach is to oxidize calcium sulfide to calcium oxide and sulfur dioxide. The calcium oxide can be recycled which greatly reduces the waste disposal problem, and the sulfur dioxide can be converted into sulfuric acid or elemental sulfur by well-known methods. However, the oxidation of calcium sulfide to calcium oxide is also difficult. Previous work [3,4] showed that the oxidation of calcium sulfide with oxygen containing mixtures at temperatures in the range of 1000.degree. to 1350.degree. C. produced both calcium sulfate and calcium oxide. Only when oxidation was conducted at 1450.degree. to 1550.degree. C. was it possible to achieve a high conversion of calcium sulfide to calcium oxide in a reasonable time [5]. Unfortunately, such temperatures are not achieved easily, and the lime would probably be dead burned and unreactive so that it could not be recycled.
Those concerned with these and other problems recognize the need for an improved process for oxidizing calcium sulfide.